FIG. 17 shows a prior art logic circuit for analog threshold processing, disclosed in pp 123 to 129 of "Nikkei Micro devices (March, 1989)". In FIG. 17, reference numeral 36 denotes a current outputting type differential amplifier and reference numeral 37 denotes a resistor having a resistance R.sub.37.
FIG. 18 shows the amplifier 36. In FIG. 18, reference numeral 38 denotes a current mirror circuit, reference numeral 39 denotes a transistor, reference numeral 40 denotes resistors having resistances R.sub.40, and reference numerals 41 and 42 denote constant current sources having current values 2Ii and Ii.
The device will operate as follows.
First of all, operation of the amplifier 36 will be described. The minus input terminal is grounded. When the input at the plus terminal is zero, only current as large as I.sub.i flows as I.sub.39. When a positive voltage is input to the plus terminal, the current I.sub.39 becomes large in proportion to I, and when it becomes 2I.sub.i, the current is saturated. Current I.sub.40 is the same current as I.sub.39 due to the function of the current mirror circuit 38, and only a current as large as I.sub.40 -I.sub.i flows as I.sub.out. The operation when the input to the plus terminal is a negative voltage is the same. From the above, the relation between the plus input V.sub.in and the output current I.sub.out is as shown in FIG. 19. This configuration is established by varying the values of R.sub.40 and I.sub.i.
The operation of the circuit of FIG. 17 will be described. The current I.sub.37 flowing through the resistor R.sub.37 is equal to the sum of the output currents of the amplifiers 36 and the voltage V.sub.out is R.sub.37 .times.I.sub.37. Therefore, when the values R.sub.40 and I.sub.i of the amplifiers are properly established, an input versus output relation of a line segment configuration, as shown in FIG. 20, is obtained. This represents a threshold processing function having an analog value as an output.
This prior art logic element for analog threshold processing includes a large number of parts and is not appropriate for integration, whereby high speed operation is impossible.